OBJECTIVES: I. Determine the maximum tolerated dose of pentoxifylline administered with hydroxyurea during a course of cranial radiotherapy in patients with glioblastoma multiforme. II. Determine the toxicity of this regimen in these patients. III. Measure PTX levels in plasma and CSF in order to assess whether therapeutic drug exposures (e.g., 0.4-2.0 mM/24 hours) can be achieved with an acceptable level of toxicity. IV. Assess the local control of glioblastoma in patients treated with this regimen. V. Determine the response of surrounding normal brain in patients treated with this regimen. VI. Determine the survival of patients treated with this regimen.